The present invention relates to steering systems which accurately and consistently restore a steering control to a selected position when the control is released, and more specifically to a steering system for moving aircraft or the like on the ground in which the aircraft proceeds in a straight direction of travel upon return of the steering control to a centered position.
In the present state of the art, aircraft moving on the ground may be steered by turning the orientation of the aircraft""s nose gear. A nose gear system generally consists of a handle mechanism with position transducers that provides handle position information to the control system that adjusts the nose gear orientation. Mechanical components of the handle mechanism are frequently subject to wear after an extended period of use. Mechanical wear may affect performance of steering systems, especially where components are designed with strict tolerances. In particular, mechanical wear, lost motion and gear backlash (i.e. the amount of play between gear teeth) may disrupt the engagement between the steering control and the nosewheel, resulting in inaccurate nosewheel control and drifting from an intended course.
The present invention provides an improved steering system. The steering system includes a main steering shaft operable from a centered or neutral position and rotatable in response to a steering force or torque applied to a handwheel mounted on the main shaft. The main shaft cooperates with a cluster of gears, such as spur gears, and a centering mechanism. The gears rotate in response to a steering force, or torque, applied to the main shaft, and the centering mechanism imparts an opposing force, or counter-torque, on the gears to return the main shaft to the original centered position when the steering force is released from the handwheel. The counter-torque is supplied by a biasing element that biases the main shaft toward the centered position so that the aircraft automatically returns to a straight course when the handwheel is released.
The present invention may be used in conjunction with a rotational variable differential transformer (RVDT) or other position-sensitive transducer. When the handwheel is turned, the RVDT monitors rotational displacement of the main shaft and converts the shaft""s angular position to an electrical signal. The signal is sent to the aircraft""s navigational system which changes the orientation of the nosewheel in accordance with the orientation of the main shaft. After the handwheel is turned and released, the centering mechanism imparts a counter-torque on the spur gears to return the main shaft and handwheel to the centered position. The RVDT monitors the rotational change in the main shaft and sends a corresponding signal to the aircraft""s navigational system to reorient the nosewheel to a centered position so that the aircraft travels in a straight line.
The present steering system returns the main shaft to its centered position accurately and consistently each time the handwheel is released. This ensures that the RVDT reads the proper orientation for centering the nosewheel. The steering system is configured to compensate for mechanical limitations, such as gear wear and gear backlash. In particular, the gears are engaged in a unified or integrated cluster, substantially preventing any gear from slipping or moving independently relative to the other gears. The spur gears are maintained in positive engagement with one another by constant loads caused by handwheel rotation and counteracting loads from the centering, mechanism. Meshed gear teeth contact surfaces do not disengage from one another when the steering direction is changed from one direction to the opposite direction. As a result, gear backlash and gear wear are minimized, allowing the centering mechanism to accurately and consistently restore the main shaft to the centered position.
The integrated gears are engaged with one another directly or indirectly, such that the gears rotate and change direction simultaneously as loads on the main shaft change. The gears are engaged at multiple interfaces within the gear cluster, minimizing the effects of wear that may occur at one location. Therefore, worn areas on an individual gear do not disrupt or affect steering accuracy or cause gear slippage. The constant engagement between gears, and the arrangement of gears as a unified integrated gear cluster, compensates for any wear and other mechanical limitations that may be present.